(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method and package for anchoring and grounding of a heat spreader that is used as part of a thermally enhanced Plastic Ball Grid Array (PBGA) package.
(2) Description of the Prior Art
The semiconductor industry has since its inception achieved improvements in the performance of semiconductor devices by device miniaturization and by increasing the device packaging density.
One of the original approaches that has been used to create surface mounted, high pin count integrated circuit packages has been the use of the Quad Flat Pack (QFP) with various pin configurations. For the QFP, closely spaced leads along the four edges of the flat package are used for making electrical connections from where the electrical connections are distributed to the surrounding circuitry. The input/output (I/O) connections that can be made to the QFP are therefore confined to the edges of the flat package, which limits the number of I/O connections that can be made to the QFP even in applications where the pin to pin spacing is small. The QFP has found to be cost effective for semiconductor devices where the device I/O pin count does not exceed 200. To circumvent this limitation, a new package, a Ball Grid Array (BGA) package has been introduced. For the BGA package, the electrical contact points are distributed over the entire bottom surface of the package thereby eliminating the restriction of having I/O connects only around the periphery of the package. More contact points with greater spacing between the contact points can therefore be allocated across the BGA package than was the case with the QFP. The contact points that are used for the BGA package are typically solder balls that have the added advantage of facilitating flow soldering of the package onto a printed circuit board.
Prior Art substrate packaging uses ceramic and plastic BGA packaging. Ceramic substrate packaging is expensive and has proven to limit the performance of the overall package. Recent years have seen the emergence of plastic BGA packaging; this packaging has become the main stream design and is frequently used in high volume BGA package fabrication. The Plastic substrate BGA (PBGA) package performs satisfactorily when used for low-density flip-chip IC""s. If the number of pins emanating from the IC is high, that is in excess of 350 pins, or if the number of pins coming from the IC is less than 350 but the required overall package size is small, or if the chip power dissipation is high (in excess of 4 Watts per chip), the plastic structure becomes complicated and expensive.
The invention addresses concerns of heat spreader anchoring and grounding of thermally enhanced PBGA packages that in addition provides advantages of electrical performance and advantages of assembly while the package meets conventional manufacturing standards.
U.S. Pat. No. 5,616,957 (Kajihari) shows a package with an anchored heat spreader.
U.S. Pat. No. 5,977,626 (Wang et al.) reveals a thermally enhanced PBGA and heat spreader design.
U.S. Pat. No. 6,032,355 (Tseng et al.) and U.S. Pat. No. 5,710,459 (Tang et al.) show other heat spreaders.
A principle objective of the invention is to provide improved anchoring and positioning of a heat spreader onto the substrate of the PBGA package by the use of pre-formed anchoring posts on the surface of the substrate.
Another objective of the invention is to provide for accurate placement of the heat spreader over the surface of the substrate.
Yet another objective of the invention is to provide for improved planarity of the heat spreader and the therewith associated substrate, thereby avoiding mold flashing or bleeding into exposed portions of the heat spreader.
A still further objective of the invention is to provide a method that prevents direct grounding between a ground pad on the surface of the substrate of the package and a lower surface of the heat spreader of the package.
In accordance with the objectives of the invention a new method and assembly is provided for anchoring the heat spreader of a PBGA package to the substrate thereof. Anchor features are made part of the PBGA package, these anchor features are provided over the surface of the substrate of the PBGA package. The anchor features align with openings created in the heat spreader stand-off, thus allowing for quick and reliable positioning and anchoring of the heat spreader over the surface of the substrate of the package.